Let’s try a thought experiment and create an incredibly simple universe. This universe is a closed universe consisting of nothing but a board with squares marked on it and a token and we are in total control. The token starts in one corner of the board and (over time) moves toward the center (forward motion of time). This is our (very limited universe) and we have control over time. After 4 time segments the token has moved 4 squares. At any given point or segment in time how many tokens is there?…one.
Now let’s move time backwards, one segment at a time until T=0 equals T=*0…at any given point or segment in time how many tokens are there?…one. Is there any difference between T=0 and T=*0? It is the exact same board, the exact same token in the exact same position, in this simple universe it is as if no “time” has passed.
But what happened?... As time passed the token “moved”...time is the ability for things (matter and energy) to move. In order to “stop” time all energy must be stopped or frozen so that things cannot move and give us the impression of the passage of time.
In our closed board and token universe, if the token never moves neither does time. The same is true for our more complicated world. If all movement stopped, if all the different kinds of energy were stopped, nothing would move and time would “stop”.
To go back in time in our universe we would have to “move” everything backwards, reverse the forces of energy acting on everything and put everything back the way it was at the point in time we are trying to get to. It sounds pretty ridiculous and it is. So let’s try the alternative that some scientists propose.
The thought is that there at every instance that there are multiple futures and it is possible to go back in time and see for ourselves…Let’s try it with our board and token closed universe. We start at T=0 and go thru the same exercise but now our token meets itself.
What happened here? We had a closed universe but now out of no-where the amount of matter (tokens) in the universe doubled.
But it doesn’t end there! Each moment in time is a possible past that we can go back to visit. Now our universe begins to look like this…
Now our supposedly closed universe in only four segments has five times as much matter in it. But time is infinitely divisible, so in reality there are now an infinite number of tokens in our supposedly closed universe.
If this were the way the universe really is then we couldn’t backup without bumping into ourselves.
Conclusion #1, Time as a dimension doesn’t exist, Einstein was able to treat time as a dimension to solve mathematical problems but that doesn’t prove the existence of time as a dimension or make it a dimension.
Conclusion #2, a good definition of time is "the method by which people can keep track of the movement of things in the present" (by comparing the movement of some things, like clocks, against other things).
The idea of time not as a reality but as a measure or concept was well known to the ancient Greeks (Antiphon, Parmenides and others) and Buddhist’s have thought of time as an illusion for thousands of years. Newton confused the concept of time by identifying it as an “absolute” and Einstein’s successful use of time as a dimension to solve physics problems ensured that the concept of time as a dimension will be a problem long into the future.
Time travel is an interesting fantasy and makes for great movies but time travel is only one way…forward, even if moving in a backward direction. :)
(Note, there is a true “force” of time (not dimension) but it deals with decay of sub atomic particles. But that is another story).
